"Jeff Saylor" wrote in message
...

I don't remember it being used before the mid 90s or so. When
did the ICAO K prefix begin to appear in front of FAA
continental USA 3 letter ID's?


I don't know when that began, but I suspect it was about the time
international air travel began to flourish after WWII. I started work at
Chicago ARTCC in 1983, they were in use by all international flights then.
You probably only became familiar with them when GPS manufacturers began
using them to distinguish between airports and navaids.